The present invention relates to disk drives and, in particular, to an apparatus and method for reducing thermal interference (thermal asperity) in magneto-resistive heads in disk drives.
Magneto-resistive and giant (G) magneto-resistive (collectively, MR) heads used in hard disk drive applications utilize a resistive sensing element to read magneticly stored data from a storage disk. The MR heads are thermally sensitive and the resistance of a MR head changes with temperature. The resistance of a typical MR head is given by the equation R=R0(1+xcex2(T-298)), where Ro equals about 50 ohms, xcex2 is the thermal coefficient of the head, and T is temperature in Kelvin. Accordingly, any variations in temperature increase/decrease the resistance of the MR head and affect the readback signal generated by the MR head as it reads data from the disk.
During operation of a disk drive, thermal interference occurs. A thermal interference event is also referred to as a thermal asperity. For additional description and discussion of thermal interference or thermal asperities, reference is made to a published article entitled xe2x80x9cElectronic Abatement of Thermal Interference in (G)MR Head Output Signalsxe2x80x9d, IEEE Transactions on Magnetics, Vol. 33, No. 5, September 1997 by Klaassen, et al., and is incorporated herein by reference. One solution proposed therein is to increase the low-frequency cutoff of a high pass filter, by switching in a resistor to lower the effective resistance of the RC high-pass filterxe2x80x94thereby increasing the cutoff frequency, when a thermal asperity is detected (the high pass filter is typically used to filter out low frequency signals in the read signal from the MR head). A significant problem with this solution is caused by the switching out of the resistor after the thermal asperity event has subsided to a certain level. This creates ringing and an unwanted offset in the read signal which may cause the loss of data. In addition, there is no control over the value of the resistor (or accuracy of the resistance value) that is switched into the circuit to lower the effective resistance of the high pass filter.
Accordingly, there exists a need for a thermal asperity recovery circuit and method that eliminates or reduces ringing and any unwanted offset in the read signal of a disk drive. In addition, there is a need for an apparatus and method that uses a programmable or variable cutoff frequency in a high pass filter used in thermal asperity recovery.
According to the present invention, there is provided a thermal event recovery circuit for filtering a read signal from a read head in a disk drive. The circuit includes a detection circuit for detecting a thermal event that affects the read signal and a filter. The filter includes a transconductance amplifier having a transconductance that varies in response to a control input. The transconductance is equal to a first predetermined transconductance in response to a first control input and equal to a second predetermined transconductance in response to a second control input, and the second control input is generated in response to the detection of the thermal event. In another embodiment, there is provided a disk drive having a storage medium, a read head for detecting data on the storage medium and generating a read signal, and the thermal event recovery circuit described above.
In another embodiment of the present invention, there is provided a thermal event recovery circuit for filtering a read signal from a read head in a disk drive. The circuit includes a detection circuit for detecting a thermal event that affects the read signal and a filter. The filter having a transconductance amplifier has a first predetermined transfer function in response to a first control input to the amplifier and a second predetermined transfer function in response to a second control input to the amplifier, and the second input is generated in response to the detection of the thermal event.
In yet another embodiment of the present invention, there is provided a circuit for detecting thermal interference and filtering a read signal from a read head in a disk drive. The circuit includes a detection circuit for detecting thermal interference affecting the read signal, a current source for outputting a first current or second current, and a filter. The filter includes a capacitive element, a fixed resistive element for generating a first resistance and a transconductance amplifier coupled in parallel with the fixed resistive element and having transconductance. The transconductance varies in response to the output of the current source, and the transconductance is equal to a first predetermined transconductance in response to the first current and is equal to a second predetermined transconductance in response to the second current. The second current is generated in response to the detection of the thermal interference.
In accordance with the present invention, there is also provided a method of reducing the affects of a thermal asperity on a read signal in a disk drive. A read signal is filtered with a filter having a first transfer function, the filter having a transconductance amplifier. When a thermal interference is detected in the read signal, the first transfer function of the filter is changed to a second transfer function by changing the transconductance of the amplifier. The read signal is filtered with the filter having the second transfer function. After a predetermined time period has elapsed after the detection of the thermal interference, the second transfer function of the filter is transitioned to the first transfer function.